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PostPosted: 03 Apr 2021, 19:30 
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I have a question related to tube socket pin # designation for a 12a_7 twin triode as shown on schematic drawings. The schematic drawing I'm working from now on my present project which is from a stereo preamp from the late 1950's has the left triode graphic representing pins 1,2 and 3 and the right graphic representing pins 6, 7 and 8. My question is if this is universal or if some schematics have this reversed?


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PostPosted: 04 Apr 2021, 13:55 
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In the 12A*7 family of tubes, the triodes are identical. As such it doesn't matter which triode is used for which stage. In guitar amps where a dual 12AX7 is typically used for the first two stages, some amps use section 2 (pins 1, 2, & 3) for the first stage and some use section 1 (pins 6, 7, & 8). And sometimes it changes when the designer updates an amplifier.

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PostPosted: 05 Apr 2021, 09:27 
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I was probably not clear in my inquiry. I was looking to see if anyone had ever seen an inconsistency in pin numbers for the two sections. For example, I've seen on rare occasions where a manufacturers schematic showed pins 1,2,3 in section 1 versus the normal pins 6,7,8. Maybe this was just a misprint on the schematic?


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PostPosted: 05 Apr 2021, 15:19 
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abartee wrote:
I was probably not clear in my inquiry. I was looking to see if anyone had ever seen an inconsistency in pin numbers for the two sections.
Oh. I misunderstood your question. I think it depends on the designer.

Here is an example from a Gibson GA-5 Amplifier where they are reversed from what you show.
Attachment:
Screen Shot 2021-04-05 at 1.13.55 PM.png

Yet here is an Epiphone Valve Jr amp with it called out like you indicate.
Attachment:
Screen Shot 2021-04-05 at 1.17.33 PM.png

Probably more dependent on the wire routing and socket installation than anything else.


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PostPosted: 05 Apr 2021, 16:28 
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So it looks like it was somewhat commonplace for them to switch what section ends up on left and right triode graphic representations. I've seen many designs from the 50s and 60s where no pin numbers are included which would make it easy for us to assume when trying to copy an old design, that left half is section 2 and right half is section 1 and cause errors in replicating the circuits.


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